Description

Motivation:

As we plan to publish an npm package for the JavaScript clp-ffi library, we need to remove standard Decompressor's boost dependency to build the necessary components properly. To achieve this milestone, we should replace boost's memory-mapped file with our own implementation. This change is first introduced in PR #387

Implementation

This PR introduces two classes necessary to wrap mmap system call:

• FileDescriptor: This is a wrapper class to a raw file descriptor. With RAII, it is easier to handle failures/exceptions where a raw file descriptor is required. To handle to potential failure in the destructor, we provide a customizable callback if users don't want to swallow the error.
• MemoryMappedFileView: This is a wrapper class to mmap system call. It maintains the mapped memory region using RAII, and provides methods to access an immutable view of the file. The memory region is unmapped in the destructor.

With the above two classes, we can replace boost's mmap support with our implementation in standard Decompressor.

Validation performed

1. Ensure the code can be successfully built for all the relevant build targets.
2. Ensure all existing unit tests are passed. The current streaming compression unit tests will call the above implementation to decompress zstd format files.
3. Add new unit tests to test memory-mapped file functionalities separately. It also tests the special case where the mapped file is empty.